1. Field of the Invention
This invention relates to a master cylinder, and more particularly to a master cylinder of the conventional type, preferable for a brake system with an anti-skid fluid pressure control apparatus which includes a hydraulic pressure pump to pressurize the brake fluid discharged from the wheel cylinder and to return it into the master cylinder.
2. Description of the Prior Art
As the prior art of the conventional type, there is known a master cylinder including: (a) a cylinder body having a cylindrical bore and an opening made in the circumferential wall, for a relief port communicating with the interior of a hydraulic reservoir; (b) a piston inserted slidably into the cylindrical bore to form a hydraulic pressure generating chamber in the cylindrical bore; and (c) a cup seal having an inner circumferential lip portion contacting with the piston, an outer circumferential lip portion slidably contacting with the inner circumferential wall of the cylindrical bore, and a base portion connecting the lip portions which are so arranged as to face to the hydraulic pressure generating chamber, whereby the cup seal cuts the communication between the hydraulic pressure generating chamber and the hydraulic reservoir through the relief port when the top end of the outer circumferential lip portions passes by the relief port. (For example, it is disclosed in the Japanese Utility Model Registration Official Opening Gazette No. 32127/1987 and No. 8931/1996.)
Although the master cylinder of the conventional type is simple in construction, in comparison with a master cylinder of the center-valve type, it has the disadvantage that the cup seal is susceptible to damage.
In the operation of the master cylinder of the conventional type, the outer circumferential lip portion of the cup seal or the circumferential portion of the base portion of the cup seal is subject to a high pressure from the hydraulic pressure generating chamber at the position of the opening of the relief port. The phenomenon is proved that the circumferential portion of the cup seal is pushed locally into the opening of the relief port. When this phenomenon is repeated at the same position of the circumferential portion of the cup seal, that part of the cup seal is subject to such damage as to appear to be bitten. Large damage leads to seal deterioration.
The occurrence of the above described damage of the cup seal is remarkable in a brake system with an anti-skid brake fluid control apparatus (ABS-Anti-lock Braking Systems, disclosed, for example in the Japanese opening gazette No. 150915/1996) which includes a fluid pressure pump to pressurize brake fluid discharged from the wheel cylinder and to return it into the master cylinder, and in which high pulsating pressure is applied to the cup seal. Irrespective of the ABS, when the master cylinder is panicly or rapidly operated to steeply raise the fluid pressure of the fluid pressure generating chamber, the above phenomenon becomes remarkable.
On the other hand, Japanese Patent Official Gazette No. 42873/1976 disclosed the master cylinder wherein the cup seal is rotated relative to the inner wall of the cylindrical bore so as to not push the same part of the outer circumferential surface of the cup seal into the opening of the relief port, and wherein slant grooves are formed in the outer circumferential surface of the cup seal.
However, in the above master cylinder, a portion of the slant groove is apt to engage with the opening of the relief port. When a high pressure is applied to the engaged part of the cup seal from the fluid pressure generating chamber, there occurs a large pressure difference between the outside and inside of the engaged groove and so that portion is so deformed as to appear to be bitten by the opening of the relief port. Thus, it was proved that the outer circumferential part of the cup seal is apt to be damaged.
Accordingly, it is an object of this invention to provide a master cylinder in which a cup seal can be effectively prevented from being submitted to local or concentric damage:
Another object of this invention is to provide a master cylinder which is durable and has long life.
In accordance with an aspect of this invention, a master cylinder includes:
(a) a cylinder body having a cylindrical bore and an opening made in the circumferential wall of the body for a relief port communicating with the interior of hydraulic reservoir;
(b) a piston slidably inserted into said cylindrical bore to form a hydraulic pressure generating chamber in said cylindrical bore; and
(c) a cup seal having an inner circumferential lip portion contacting with said piston, an outer circumferential lip portion slidably contacting with the inner circumferential wall of said cylindrical bore, and a base portion connecting said lip portions, which lip portions are so arranged as to face to said hydraulic pressure generating chamber, whereby said cup seal cuts the communication between said hydraulic pressure generating chamber and said hydraulic reservoir though said relief port when the top end of said outer circumferential lip portions passes by said relief port, the improvement including means for rotating said cup seal relative to said cylindrical bore with the movement of said piston is provided at portions thereof other than portions of said cup seal facing to the inner wall of said cylindrical bore.
In accordance with another aspect of this invention, the master cylinder includes:
(a) a cylinder body having a cylindrical bore and an opening made in the circumferential wall, of the body for a relief port communicating with the interior of hydraulic reservoir;
(b) a piston slidably inserted into said cylindrical bore to form a hydraulic pressure generating chamber in said cylindrical bore; and
(c) a cup seal having a base portion supported at the back by a flange portion of said piston, and an outer circumferential lip portion extending towards said hydraulic pressure generating chamber from the front side of said base portion, and contacting with the inner wall of the cylindrical bore whereby said cup seal cuts the communication between said hydraulic pressure generating chamber and said hydraulic reservoir through said relief port when the top ends of said lip portions pass by said relief port, the improvement including the inner diameter of said base portion is so large that a gap is formed between the inner circumferential surface of said base portion and the outer circumferential surface of the axial portion of said piston fitting to said base portion, and means for inducing rotational force to said cup seal relative to the inner wall of said cylindrical bore with the relief of the fluid pressure of the fluid pressure generating chamber or reducing the contact pressure on said piston and thereby cause said cup seal to rotate, and said means is provided on said cup seal in portions thereof other than the outer circumferential surface of said base portion and outer lip portion.